Evening all,
I have recently been experiencing an issue with the expansion tank on my blacktop zetec.
I had to replace the expansion tank a little while back as it developed a split.
I replaced with a Renault Clio unit, all was well until I started to get coolant being expelled via the cap.
I have changed the cap to a higher pressure one, however I am still getting coolant coming out of the cap when the pressure builds up.
I went out for a driver earlier, it was fine for around 40 mins, until I slowed down in traffic and the temp increased.
I am not seeing the engine overheat, it just seems to build up pressure after a period of time.
Any ideas?
P.S. it is a brand new crate engine, with about 350 miles on it.
thanks,
Liam

I believe you may need to fit an idler pulley on the auxiliary belt to get the water pump to rotate in the correct direction? It may be you have an
air lock in the cooling system ? Are you using a heater or blanked off heater pipe connections? How is the cooling fan controlled?

EDIT. Re reading you have already tried a higher pressure cap. There seems to be an issue here, are you overfilling the header tank? If not, and
definitely no overheating, I would be getting the coolant "sniffed" to rule out HGF.

Re possible overheating, how are you checking that? has your gauge been calibrated? or used an IR thermometer when the issue occurs?

The pressure at which a cap blows at depends on how much the spring is compressed.

When I made my overflow header tank I copied the cap recess from the Coolman rad that I had bought from Robin Hood,
as it was then.
I bought a QH cap marked up at 11 psi which is correct for the 907 engine but when pressure tested it blew at 4 psi.
As a quick fix for test purposes I placed a 3mm thick rubber washer in the tank cap recess to compress the spring more
and hey-presto it blew at 12 psi.

I have a mate ( just the one ) whose 1600 CF had a similar problem. New cap fitted and a 3 mile test drive. Great.
Didn't drive the car all winter and the first long run in the spring and it boiled. Turned out to be a shorter cap.

HTH
Paul G

The one and only member of the Suttol Owners Club. Now also the MX5 Owners Club. A few more in that one.

thanks for all your suggestions.
I really hope it isn't a head gasket failure as it is a brand new engine with very little mileage. There isn't any milky substance in the
oil.
The old tank just had a normal screw top cap, no pressure valve in it, which resulted in no issues, apart from it splitting (it did look old and
knackered).
Do you think I should try and use a header tank with no pressure valved cap? There is a pressure cap on top of the rad

Is that a blank cap on the radiator or another pressure cap. You should only have one pressure cap in the system, with 2 it won't work
correctly.

This is how mine is plumbed and its worked perfectly since 2002 with both Silvertops and a Blacktop with one exception. Fitted a cheap (£150 from a
burnout) 2 litre which initially actually ran beautifully but the headgasket failed after a few hundred miles, expansion pressurised and made a right
mess. New headgasket and 1/2 day in the garage sorted it.

You say you hope its not the head gasket since its a brand new engine with low mileage. Personally I would not expect a problem with the engine but be
aware that the last of these Zetecs was actually built in 2004 and they have been stood in warehouses ever since. So whilst its low mileage it coming
up 13 years old at least.

The cap on the radiator needs to be a blanking car and not a pressure seal cap. This way you should then have a continuous flow of coolant from the
small bore pipe back to the header tank. Don't think about the head gasket until you have completely eliminated cooling system issues. I
convinced my self I had a blown head gasket on my marlin when It was infacy my header tank set up and a massive airlock.

Just been out to the car to investigate further...
The cap on the rad is in fact a pressure cap! So I am hoping this is where the problem lies!
Next question is... is it easier to source a blanking cap for the rad or get a not pressure cap for the tank/new tank?